The present invention relates to a power conversion system, particularly to the reduction of high-frequency current that is caused as the switching frequency of semiconductor switching devices have become higher.
According to a conventional technique disclosed on a Japanese Laid-Open Patent Publication No. Hei 10-136654 (1998), it is suggested that the common mode current through the grounding circuit is reduced by detecting the common mode current by a zero-phase-sequence current detector and feeding back the common mode current directly to the inverter by means of a current compensating current source using a semiconductor device.
According to a noise control system disclosed on a Japanese Laid-Open Patent Publication No. Hei 5-22985 (1993), it is suggested that the ratio of the common mode current circulating into the inverter to the common mode current flowing out to the grounding circuit is changed, and hence noise is reduced, by directly connecting, with a conductor such as a grounding lead, the grounding terminal of a control unit including the inverter and the grounding terminal of the load such as a motor
According to a voltage-type inverter outputting n-phase AC voltage (xe2x80x9cnxe2x80x9d is a natural number) disclosed on a Japanese Laid-Open Patent Publication No. Hei 11-122953 (1999), it is suggested that the common mode current through the grounding circuit is reduced, and hence noise is reduced, by forming a common mode current circuit for bypassing the grounding circuit wherein the power is supplied through xe2x80x9cnxe2x80x9d number of coils of a zero-phase transformer having xe2x80x9cn+1xe2x80x9d number of coils and the neutral point of a capacitor connected to the n-phase AC output terminal is connected to the DC side of the inverter via the remaining coil of the zero-phase transformer.
The inventors found that with the conventional technique using a common mode choke, however, no effect is expected unless the inductance of the common mode choke is higher enough as compared to the inductance of the power line between the inverter and the load (motor) or the inductance of the grounding portion. As a result, they found a problem that if the power line from the inverter to the load is longer, the common mode choke increases in its capacity and size, and accordingly in its cost.
In case, the conventional technique for detecting the common mode current by a zero-phase-sequence current detector and feeding back the common mode current from the current compensating current source to the inverter, they found that the zero-phase current detector must be faster in speed and higher in accuracy and besides, in order to obtain satisfactory effect of the technique, the semiconductor device used for the current compensating current source must also be larger in capacity.
The inventors also found that with the conventional technique for directly connecting the grounding terminal of the inverter and that of the load with a conductor in order to reduce the common mode current flowing out to the grounding circuit, not a little common mode current flows through the grounding current, and hence radiated noise is caused, unless the impedance of the common mode current feedback line to which the grounding terminal of the inverter and that of the load are connected is lower enough as compared to the impedance of the grounding circuit. In particular, they showed that if the impedance of the grounding circuit is low, very little noise reduction effect is expected.
The inventors found the following facts. With the conventional technique for forming a common mode current circuit wherein the neutral point of a capacitor connected to the n-phase AC output terminal is connected to the DC side of the inverter via one of the coils of the zero-phase transformer, rush current is likely to be caused in the capacitor upon switching the inverter. An inductance must be installed between the inverter and the capacitor to prevent this rush current, resulting in increased size of the system and hence increased cost. Besides, since the capacitor is connected to the inverter and the zero-phase transformer is used to eliminate the zero-phase-sequence voltage, the exciting inductance of the zero-phase transformer must be higher enough as compared to the inductance of the wiring. That is, the system must be larger, involving some cost increase.
An object of the present invention is to suppress, using a simple system, the high-frequency noise interference caused by the common mode current.
The aforementioned problems can be solved by the following means.
According to an aspect of the present invention, when the power is converted by switching a semiconductor switching device, high-frequency current corresponding to the switching frequency is generated and the high-frequency current flows in the loop of the grounding circuit comprising of the power line and the ground with the aid of the floating capacity between the power converter and the ground and between the load and the ground. The high-frequency current, which is called the common mode current because it flows commonly in the power lines corresponding to the number of output phases of the power converter, is apt to cause high-frequency noise interference, including radiated noise and conducted noise, to the peripheral devices.
Another feature of the present invention is a power conversion system with high-frequency noise interference suppressing means, wherein the output power line of a power converter and the common mode current circulation line is wound together on a magnetic core and each end of the common mode current circulation line is connected to the grounding portion of the power supply of the power converter and the grounding portion of the load. The feature is capable of suppressing the high-frequency noise interference caused by the common mode current, employing a simple constitution.
Another feature of the present invention is a power conversion system with high-frequency noise interference suppressing means; wherein the output power line of a power converter and the first common mode current circulation line are wound together on a magnetic core and the input power line and the second common mode current circulation line on another magnetic core, each end of the first common mode current circulation line is connected to the grounding portion of the power converter and the grounding portion of the load, and each end of the second common mode current circulation line is connected to the grounding portion of the power converter and the grounding portion of the power supply of the power converter. The present invention is capable of suppressing the high-frequency noise interference caused by the common mode current, employing another simple constitution. The present invention is not limited to the above description and further explained below referring to drawings.